In general, a motor with two rotors should have two corresponding stators, and each of the stators should have coil to drive the corresponding rotor; however, the above design will significantly increase the size, weight and cost of the motor; on the other hand, the above design cannot provide high torque in low speed and wide speed region.
However, in-wheel motor of electric motor, integrated starter generator and other similar applications are required to be of small size and light in weight; besides, they also need to provide high torque in low speed and achieve wide speed region; therefore, it is a great challenge to design a motor capable of meeting the above requirements.
Currently, many different motors have been developed for the above applications. For example, US patent publication No. 20130093276 provides a double-stator/double-rotor type motor; U.S. Pat. No. 7,030,528 provides a dual concentric AC motor; Taiwan patent publication No. 6924574 provides a dual-rotor, radial-flux, toroidally-wound, permanent-magnet machine. However, the above motors still have a lot of shortcomings to be overcome.